A silane coupling agent has two or more different functional groups in its molecule, and acts as a mediator for joining an organic material and an inorganic material together although such organic and inorganic materials can be hardly joined together in general. One of the functional groups is a hydrolyzable silyl group, and in the presence of water, forms a silanol group. As a result of a reaction with a hydroxyl group on the surface of an inorganic material, the silanol group forms a chemical bond with the surface of the inorganic material. The other or another functional group is an organic reactive group such as a vinyl, epoxy, amino, (meth)acryl or mercapto group, which forms a chemical bond with an organic material such as a synthetic resin. Making use of these properties, silane coupling agents are widely used as modifiers for organic and inorganic resins, adhesion aids, various additives and the like.
Among such applications of silane coupling agents, their application as pressure-sensitive adhesives is representative. For pressure-sensitive adhesives to be used upon bonding liquid crystal cells and optical films together, for example, there is an outstanding demand for still higher adhesion performance as a result of the move toward greater and wider liquid crystal displays (LCD).
In the case of LCDs, there is an ever-increasing move toward larger panels, contrary to the early-stage expectation that a size increase beyond 20 inches would be difficult. Major manufacturers have heretofore concentrated their efforts on the manufacture of small panels of 20 inches and smaller. Responsive to such a trend in recent years, however, they are now actively introducing latest technologies to expand their product range to larger sizes of 20 inches and greater.
As mentioned above, there is a trend toward larger glass panels for use in combination with various optical films upon manufacture of liquid crystal display panels. If a defective product occurs at the time of initial bonding of an optical film to a liquid crystal cell, the optical film may be removed from the liquid crystal cell, and then, the liquid crystal cell may be washed to permit its reuse. If a conventional pressure-sensitive adhesive having high adhesiveness is used, this high adhesiveness makes it difficult to remove the optical film upon its separation due to the high adhesiveness force and moreover, is accompanied by a high potential risk of breaking the costly liquid cell. As a consequence, the use of such a conventional pressure-sensitive adhesive leads to a significant increase in manufacturing cost.
Keeping in step with the move toward larger LCDs, attempts have, therefore, been continued to develop high-function pressure-sensitive adhesives capable of satisfying various adhesion properties such as adhesiveness and rework capability. For example, Japanese Patent No. 3022993 proposes an epoxysilane-containing, acrylic pressure-sensitive adhesive composition for the purpose of providing a polarizer excellent in durability under an environment of high temperature and high humidity.
Further, JP-A 8-104855 proposes a pressure-sensitive adhesive composition containing an acrylic polymer and a compound which has a β-ketoester group and a hydrolyzable silyl group, not only to permit bonding a polarizer on the surface of a substrate with good adhesive force but also to permit removing the polarizer from the surface of the substrate as needed without giving damage to the substrate or allowing the adhesive to remain.
It is described that owing to the inclusion of such a silane compound, the substrate and the polarizer can retain adequate adhesive force of such a level as required in an actual use environment, the adhesive force does not become excessive by heating or the like, and the polarizer can be readily removed without giving damage to the liquid crystal device.
As performance required for a pressure-sensitive adhesive to support the move toward larger LCDs, it is necessary not only to produce low initial adhesive force upon bonding to glass and to assure excellent rework capability but also to develop high adhesive force under high temperature and high humidity. Otherwise, there is a potential problem that bubbling, separation and/or the like may take place to lower the durability.
JP-A 8-199144 proposes a technology that incorporates a curing agent in an acrylic resin which is obtainable by polymerizing an acrylic monomer in the presence of a silane compound to provide a pressure-sensitive adhesive composition that does not undergo much variations with time in cohesive force and adhesive force even under high temperature and high humidity and is also excellent in adhesive force for curved surfaces.
It is described that owing to the incorporation of the silane compound, the substrate and the polarizer can retain adequate adhesive force of such a level as required in an actual use environment, the adhesive force does not become excessive by heating or the like, and the polarizer can be readily removed without giving damage to the liquid crystal device.
However, a pressure-sensitive adhesive composition is considered to be preferred when it is high in adhesive force, free from bubbling or separation and excellent in durability rather than when it does not undergo much variations with time in cohesive force and adhesive force even under high temperature and high humidity. In other words, it is considered necessary to show adequate initial adhesive force of such a level as permitting removal of a polarizer in an initial stage after its bonding to glass but, as time goes on, to be enhanced in adhesive force and to retain stabilized adhesive force because it becomes no longer necessary to remove the polarizer.
As a pressure-sensitive adhesive low in initial adhesive force and excellent in rework capability, and after bonding, enhanced in adhesive force under high temperature and high humidity and excellent in durability over long term, JP-T 2008-506028 proposes an acrylic self-sensitive adhesive composition containing a silane coupling agent having a urethane functional group and a pyridine functional group.
However, the silane coupling agent is obtained by reacting an isocyanatosilane and 2-pyridinol in the presence of a catalyst, and a hydroxyl group of 2-pyridinol non-selectively reacts to both the isocyanato group and hydrolyzable silyl group of the silane. Accordingly, the silane coupling agent does not have such a single structure as the disclosed silane, and is insufficient in the improvements of various self-adhesion properties.
Under the foregoing circumstances, it has been desired to develop a pressure-sensitive adhesive which has rework capability in an initial stage and retains high adhesiveness force under high temperature and high humidity.